1,789 research outputs found
Hybrid Neural Networks for Frequency Estimation of Unevenly Sampled Data
In this paper we present a hybrid system composed by a neural network based
estimator system and genetic algorithms. It uses an unsupervised Hebbian
nonlinear neural algorithm to extract the principal components which, in turn,
are used by the MUSIC frequency estimator algorithm to extract the frequencies.
We generalize this method to avoid an interpolation preprocessing step and to
improve the performance by using a new stop criterion to avoid overfitting.
Furthermore, genetic algorithms are used to optimize the neural net weight
initialization. The experimental results are obtained comparing our methodology
with the others known in literature on a Cepheid star light curve.Comment: 5 pages, to appear in the proceedings of IJCNN 99, IEEE Press, 199
The red blazar PMN J2345-1555 becomes blue
The Flat Spectrum Radio Quasar PMN J2345-1555 is a bright gamma-ray source,
that recently underwent a flaring episode in the IR, UV and gamma-ray bands.
The flux changed quasi simultaneously at different frequencies, suggesting that
it was produced by a single population of emitting particles, hence by a single
and well localized region of the jet. While the overall Spectral Energy
Distribution (SED) before the flare was typical of powerful blazars (namely two
broad humps peaking in the far IR and below 100 MeV bands, respectively),
during the flare the peaks moved to the optical-UV and to energies larger than
1 GeV, to resemble low power BL Lac objects, even if the observed bolometric
luminosity increased by more than one order of magnitude. We interpret this
behavior as due to a change of the location of the emission region in the jet,
from within the broad line region, to just outside. The corresponding decrease
of the radiation energy density as seen in the comoving frame of the jet
allowed the relativistic electrons to be accelerated to higher energies, and
thus produce a "bluer" SED.Comment: 5 pages, 4 figures, MNRAS Letters, in pres
The NuSTAR view on Hard-TeV BL Lacs
Hard-TeV BL Lacs are a new type of blazars characterized by a hard intrinsic
TeV spectrum, locating the peak of their gamma-ray emission in the spectral
energy distribution (SED) above 2-10 TeV. Such high energies are problematic
for the Compton emission, using a standard one-zone leptonic model. We study
six examples of this new type of BL Lacs in the hard X-ray band with the NuSTAR
satellite. Together with simultaneous observations with the SWIFT satellite, we
fully constrain the peak of the synchrotron emission in their SED, and test the
leptonic synchrotron self-Compton (SSC) model. We confirm the extreme nature of
5 objects also in the synchrotron emission. We do not find evidence of
additional emission components in the hard X-ray band. We find that a one-zone
SSC model can in principle reproduce the extreme properties of both peaks in
the SED, from X-ray up to TeV energies, but at the cost of i) extreme electron
energies with very low radiative efficiency, ii) conditions heavily out of
equipartition (by 3 to 5 orders of magnitude), and iii) not accounting for the
simultaneous UV data, which then should belong to a different emission
component, possibly the same as the far-IR (WISE) data. We find evidence of
this separation of the UV and X-ray emission in at least two objects. In any
case, the TeV electrons must not "see" the UV or lower-energy photons, even if
coming from different zones/populations, or the increased radiative cooling
would steepen the VHE spectrum.Comment: 13 pages, 2 figures. Version accepted for publication in MNRAS. Fig.
2 corrected for a small plotting erro
The Space Object Ontology
Achieving space domain awareness requires the
identification, characterization, and tracking of space objects.
Storing and leveraging associated space object data for purposes
such as hostile threat assessment, object identification, and
collision prediction and avoidance present further challenges.
Space objects are characterized according to a variety of
parameters including their identifiers, design specifications,
components, subsystems, capabilities, vulnerabilities, origins,
missions, orbital elements, patterns of life, processes, operational
statuses, and associated persons, organizations, or nations. The
Space Object Ontology provides a consensus-based realist
framework for formulating such characterizations in a
computable fashion. Space object data are aligned with classes
and relations in the Space Object Ontology and stored in a
dynamically updated Resource Description Framework triple
store, which can be queried to support space domain awareness
and the needs of spacecraft operators. This paper presents the
core of the Space Object Ontology, discusses its advantages over
other approaches to space object classification, and demonstrates
its ability to combine diverse sets of data from multiple sources
within an expandable framework. Finally, we show how the
ontology provides benefits for enhancing and maintaining longterm
space domain awareness
The BMW (Brera-Multiscale-Wavelet) Catalogue of Serendipitous X-ray Sources
In collaboration with the Observatories of Palermo and Rome and the SAX-SDC
we are constructing a multi-site interactive archive system featuring specific
analysis tools. In this context we developed a detection algorithm based on the
Wavelet Transform (WT) and performed a systematic analysis of all ROSAT-HRI
public data (~3100 observations +1000 to come). The WT is specifically suitable
to detect and characterize extended sources while properly detecting point
sources in very crowded fields. Moreover, the good angular resolution of HRI
images allows the source extension and position to be accurately determined.
This effort has produced the BMW (Brera Multiscale Wavelet) catalogue, with
more than 19,000 sources detected at the 4.2 sigma level. For each source
detection we have information on the X-ray flux and extension, allowing for
instance to select complete samples of extended X-ray sources such as candidate
clusters of galaxies or SNR's. Here we present an overview of first results
from several undergoing projects which make use of the BMW catalogue.Comment: 7 pages, 6 postscript files, 2 gif images, to appear in the
proceedings of the conference "Mining the Sky", August 2000, Garching,
German
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